Lab Rat
by Parabola Beam
Summary: The cornered rat bites the cat. But who's really the rat? Rin/Len, takes place during Karakuri Manji Burst. Hard citrus content in future chapters, mind games, dark themes.
1. Day One

**Lab Rat**

Day One

Len scrutinized her carefully. She was far and above the most important prisoner they'd taken, and by the same measure, the most deadly, with a long and bloody history behind her.

Keeping her here in solitary was a risk he'd take, for the greater good. Someone like her undoubtedly had information that would be helpful to them, and he'd spend as long as it took to get it out of her.

"Hands on the wall. Spread your legs."

She complied, too easily, but not before shooting him a mischievous grin. Len was wary: it was no secret that she was as dangerous as she was beautiful, and she was very, very beautiful. He would not, could not make a single mistake around her or she wouldn't hesitate to exploit it.

"Do you really think I could hide more weapons wearing this little?" He could hear the smile in her words, and he knows what she's trying to do to him.

It's not going to work. His hands settled on the silky fabric of her mini kimono, feeling carefully along her lithe, slender body. Waist, hips, thighs. She moaned, much too comfortable for his liking. He felt his way down her ornately patterned stockings, one lean, firm leg at a time. Finding nothing, he straightened up and looked her carefully up and down.

"You're surprisingly ill-equipped."

"If you're looking for the weapon," She drawled over her shoulder, her tone bordering on insulted, "you're looking at her."

"Hands up. Face me." She obeyed without complaint. "For a weapon, you're sure being well-behaved."

"I'm used to being touched..." A sultry purr coated her words as she took a confident step forward, "...admired..." Another step forward, with a coquettish sway of her hips, and they're close enough for him to feel her breath on the nape of his neck. He stands his ground. Just this once, he'll allow her the proximity, so he can rebuff her and make sure she gets the message. Tender, full lips touch his ear before she sighs into it, cheek to cheek, "...handled."

He reaches up with his hand, pinching her jaw and pushing her into the wall.

"Are you finished?"

She's looking right back at him with that one uncovered eye, the color of bloody wines and thorny roses.

And then, Len makes his first mistake.

He looks at her lips.

The tip of her tongue slides slowly between the edges of her teeth. The movement is casual-deliberate, the sort of thing that could be mistaken for an unintentional mannerism, but he knows better.

For a terrible moment, he wants to kiss those lips, those arrogant, sinister, perky lips. They're close enough to taste and he can picture the way her tight, muscular body could feel against him, if he were only to seal her to this wall and trap her under his touch.

She smiles and he knows she knows exactly what he's thinking.

He's the one who's trapped.

**A/N: **I've always been intrigued by Rin and Len's relationship in Karakuri Manji Burst, so I finally decided to start this. This idea has been waiting patiently in my head to be written, so now I'm gonna do it! This'll be my first time writing them so I apologize in advance if I fall short of expectations. XD Thank you for reading the first chapter, and if you felt like leaving a review I'd be pleased as punch. 8D


	2. Day Two

**Day Two**

Rin sighs for the umpteenth time as she tries to get comfortable in the chair she's handcuffed to. At least the pointless questioning is over for the day.

She is very bored. This is unfortunate for dear big brother.

"I am thirsty." Rin announces. "May I drink something?"

He pushes off from the wall he was leaning on, the wall where he ever-so-vigilantly watched her sit and fidget. Truth be told, she wasn't even expecting him to do what he did, to actually go to the table and pour her a cup of water. She'd been hoping for him to refuse so she could pester him for a prolonged duration of time.

Len brings her the water, holding the cup near her lips. She sits up and drinks daintily, swallowing the cool mouthfuls and smiling up at him.

"This was very sweet of you."

"We aren't savages. And as long as you continue to be forthcoming with information, you are a guest."

A name here, a name there. Cogs in the wheel. Replaceable. A small price to pay to allow Len to return to his superiors with information, to give the illusion that his station here served a purpose other than to satiate her boredom.

To allow him to stay near her.

"Hmm, hm..." Rin hummed thoughtfully. "...You must be bored though." His eyes flickered down towards her. "Stressed?" She purred slowly, hissing her s's softly. He raised an eyebrow. "Are you...interested in unwinding a bit?"

"...I can't imagine what you could mean by that-" He was interrupted by Rin's coy giggle.

"More like 'won't'. Come on now..." She cooed, slipping her foot out of her shoe and slowly running it up and down the inside of his leg. His eyes widened, but he didn't pull away. "...There aren't too many ways for a boy like you and a girl like me to have fun in here."

"I will not-" His voice came slow and emphatic, but Rin could hear the heavy swallow that preceded his speech, "-dishonor myself or anyone else here by laying with you."

She giggled sultrily. It was so cute when he tried to be firm, and his resistance only made her want to erode and corrupt his ever-pure sense of honor all the more. Watching him melt like warm butter for her would be her prize, the distinction of reducing her own ethical, honest, principled, pedigreed older brother into her very own simple-minded _boy toy_.

"Then why not just lay back..." She whispered, to make him listen, "...and let me do all the work?" Her foot ascended further up his leg, nudging his package through the fabric of his pants. Len's legs lock, and Rin smiles: she remembers, she does, how he gets when he's afraid, how he just goes rigid and becomes so very vulnerable.

Helpless.

"I've been taught how to behave for men." She continued stroking him with her foot idly as she talked, batting her eyes at him cattily as she felt him swelling to life. "And I know what someone like you wants out of someone like me."

"Is that so." Try as he might to sound assertive, the fact of the matter was that he was not pulling away from her; Rin knew full-well she was still in charge. "And what would that be?"

"You just need a playmate is all. And besides, we might have more in common than you think."

Or remember.

The corners of his mouth twitched. "I doubt that."

Rin giggled with bemusement. "Well when you get right down to it...we're both just someone else's toys. But..." She pulled her leg back and crossed it over the other. "...why not make me _your_ toy?" When Len didn't answer right away, she leaned forward with a whisper. "No one's watching. No one has to know."

"I would know. And so would you."

Funny. He didn't actually refuse.

"Are you saying that you care what I think about you?" Rin smirked. A startled gasp came from her throat as his fingertips curled underneath her chin and forced her to look at him, eye to eye.

"I'm going to make this as clear as possible, little girl-"

"'Little girl'?" Rin laughed derisively, "Why, you can't be that much older than me."

There was a flutter of something dark in his expression that unnerved and thrilled her. "Shut up." His lips curled into a defiant sneer. "I'm not interested in any games you want to play. Of any nature."

"If you can't convince yourself-" She articulated evenly, slowly, "-then you have no chance against me."

He wasn't fooling anyone with that tone of voice, least of all her.

Even if he'd buried his memories of her, she could never forget him if she'd tried.

She delicately pulled her chin from his hand. No sooner did she slip away did he take it again, bringing their faces closer. Rin held very still. She knew he wouldn't. Not yet.

And she was right. He didn't.

Much too pleased with herself, she eased forward just slightly, until their lips hovered too close a space together to be measured. Rin parted hers-

-and simply breathed.

_I want to, and I could, but I won't yet._

The warmth mingled between them, humid, soft. There was something delicate about it, something about the moment that felt old and new at once.

Rin just can't, _can't_ think about the past, and they couldn't be at a stand-still indefinitely. She bridged the distance and softly sank her teeth into his bottom lip, feeling the low vibration of a guttural moan spilling forth. She hummed breezily in response, pushing her teeth in with bruising force, tasting his trepidation in the way his breath hitched. Just before he was about to deepen the contact, she broke away, leaving him yearning, aching for her.

Rin smiled at his betrayed, baited expression. "I won't take your honor just yet..." She smirked mischievously as he turned away and started to leave, "...or your virginity."

He stopped and glanced over his shoulder with a hateful scowl before slamming the door and leaving her alone with her thoughts.

She did enjoy getting under his skin.

But she'd rather get on top of it.

**A/N: **Sorry for the wait! The plot shall thicken more in coming chapters~. I'm really enjoying writing these two, especially Rin. XD I hope you enjoyed the chapter, thank you so much for reading and I'd love it if you felt like reviewing!


	3. Day Three

**Day Three**

Len did not want to take a break.

He didn't trust anyone else watching her, least of all other men in the division that didn't possess such strict morals as himself.

She was his prisoner. His enigma to decode.

But top brass was irrefutable. Len supposed he should've been honored, that he was valuable enough an asset to keep in good condition by taking such reprieves, and there was a time where he would've been.

If he could get her words out of his head.

Toys.

The simple word makes him so frustrated now that he's putting his all into sparring practice. Neru is his partner today, simply because she's the best with blade work, and if he's going to be forced to take a break, he will settle for nothing less than making it the most productive he possibly can.

She has a temper, a natural bloodlust that has been honed to a deadly degree of control, but today, he's giving her a run for her money. There's a constant, tonal ringing sound of their blades meeting again and again as they dance in near-unison. Neru smiles, obviously pleased with the challenge, and for some reason Len gets the feeling that she's not even using her full strength, and anger spikes hot inside.

He forgets himself. The swings of his blade intensify, savage, and the smile disappears from Neru's face as she puts all her energy into parrying and blocking his relentless assault.

"E-Enough-" She gasps out, and he knows it must be serious, if she of all people is bowing out, but he goes too far, loses control. His blade glances across her collarbone, her shoulder, and she shrieks behind grit teeth, clutching her wound.

"...The fuck?" Neru spits harshly, glaring at him as she sinks to a kneel, pain etched into her face.

"N-Neru-" He stutters many apologies, quickly sheathing his weapon and helping her to her feet so he could guide her to the medical ward. Begrudgingly, she accepted his assistance.

...

"You again, Neru?" Dell sighed. "Look, Haku's not around right now to kiss your boo-boos, but if you need another band-aid-" The cigarette dropped from the mouth of the irate doctor in surprise as he caught a glimpse of Neru's wound, "-well god damn."

"Thanks for the concern, prick." She grimaced, laying down on the cot at his behest, Len assisting her as much as she would tolerate.

"Way to be, Neru. Haku's gonna have a heart attack when she sees you-"

"It was my fault." Len explained quickly, displeased with himself, "Sparring practice. I should've been more careful."

"Yeah yeah." Dell barely acknowledged him, leaning over Neru and inspecting her shoulder critically. "Lucky you. Stitches." Neru groaned in great dismay. "Oh can it. I'll numb you down and get Haku in here to sew you up before anything important falls out."

Convinced that she was in good hands, Len took his leave of them and retreated from the medical ward to return to his practice, what little of it he could now do without a partner. His pace slowed as he neared the interrogation room, where his prisoner was being kept presently.

A sense of bitter disdain washed through him. Any time others spent on her was a waste. She'd gained a quick reputation for giving everyone but Len nothing but the cold shoulder and it would be in everyone's best interest if he were returned to her watch.

He was about to move on when a loud noise from inside the room caught his attention, like that of a chair being knocked over, followed by a cold, smooth voice.

"I know what sort of work you do." Gakupo? "When you get right down to it, you're just a trashy little sex kitten, no more, no less. All I want is a little of what you're giving out-"

"-And I'm telling you it's not for you." Her voice is venomous, hostile. "Back off."

"Maybe I'm telling you you don't have a choice-"

"Stop-" Her voice is actually wavering now, "-I won't ask twice-"

"You're in no position to be making demands, my dear-"

Having heard more than enough, Len threw open the door, enraged, in time to see him receiving the business end of a nasty headbutt. Gakupo reeled back, releasing her, and she fell to the ground unceremoniously, cringing defensively as best as she could with her hands still behind her back. He observed that her obi had been forced looser and that the skirt of her mini kimono had been hiked high over her thigh. She curled her legs closer and snarled hatefully in Gakupo's direction. Her expression was positively feral.

"Len!" Gackupo sputtered, flecks of blood spraying from his lips. He was bleeding rather badly, and upon closer inspection, Len realized he was now missing a tooth. "Your little girlfriend is out of control, I was interrogating her-"

Len heard him. He did not ignore him, but he also did not respond. He immediately went to help her to her feet, holding her against him protectively.

She was quaking. Fear? Anger? Both?

"And what, you had to take off her clothes to question her?" Len countered coldly. Before Gakupo could answer, she slumped against him, and Len might've thought it to be one more of her tricks if he hadn't seen the blood matting her bangs together. He held her steady and pushed her hair aside; blood was flowing from her forehead down and over the bridge of her nose, from the wound that must've been made by the impact with Gakupo's tooth.

"Mh...My head..." She drawled dizzily, biting her lip until it was white with the effort to remain standing. Blood, and blunt trauma. He picked her up, cradling her without meaning to. She's so very small all of a sudden, and he can't place this terrible wave of déjà vu he's experiencing, but he's not going to let any more danger come even close to her, and the feeling is like an intensely violent high.

He wasn't going to fail. Not again.

Just then, Luka came in, weapon at the ready.

"The door was open." Luka surveyed the room, "What's happened?"

"Gakupo broke procedure with the detainee and injured her in the midst of an attempted sexual assault. Don't deny it-" Len barked as the man in question shook his head and opened his mouth to protest, "-I heard everything!"

If looks could kill, Gakupo would've died from the glare Luka shot him. "Is this true?" When he didn't respond, she closed the distance between them and planted her hand on his throat, lifting him against the wall. "Answer me, maggot, and don't lie!"

"Yhh...Yes-" He coughed. Luka wrenched him forward, throwing him to the floor. She planted her boot firmly in his chest

"We are better than this. What's worse, she's our only lead. You are an insufferable disgrace." She spat right into his face, and Ruko smirked, obviously enjoying the show. "I hereby strip you of your rank, titles, and privileges. You will submit your badge and your weapons, and never return." She turned to Len. "Take her to the infirmiry to get that looked at, quickly. You're not going to want to see this anyway."

Upon being granted leave, Len immediately made for the medical ward once more, leaving Gakupo's harrowed screams in the hall behind him. His prisoner is quiet but conscious in his arms, and he doesn't know what to say to her, so he says nothing.

...

"You again?" Neru winces as Haku weaves the needle in and out of her skin. "No one's safe around you, huh?" Haku whispers something to her hastily and Neru's eyes widen. "...Gakupo? Really?" Haku nods and gestures for her to keep her voice down.

Bad news travels fast. He ignores them in favor of getting the prisoner to the unoccupied doctor.

"Long time no see." Dell remarks sarcastically, motioning towards the spare cot. Len lays her down, and she is on her best behaviour, even when they remove her handcuffs to take her pulse. The doctor cleaned her wound and snugly secured some gauze over it; the bleeding had already slowed quite a bit. He supplied a few painkillers for her, which she took gratefully.

"It's not serious then?" Len inquired.

Dell gave a curt shake of his head. "Nope, she'll be just fine. Was probably just the shock that got her more than the injury, just a little wooziness." He quirked an eyebrow at the patient. "Can you walk? Yes, you can, you're a tough cookie." He sighed and reached for a cigarette, grumbling to himself. Len cuffed the prisoner's hands in front of her and helped her to her feet, guiding her back to her usual cell.

...

The walk back had been uneventful. He didn't like it, he decided, when she was eerily silent like this. They enter the room and after he closes the door, but before leading her to her chair, he speaks.

"I apologize for what you experienced today. None of it should've hap-" She lifts her bound wrists, looping her arms around his neck, and pulls.

She kisses him.

It is not breathing. It is not a nibble. It is a full, deep kiss, and he takes no pride in the fact that were he not too shocked to do anything, he'd seize her waist and return it and maybe not let her go.

But she lets go, smirking against his shocked, parted lips. In a way, it is mercy.

"You've saved me from a rather nasty situation." Her fingers tease the nape of his neck, the mischievous, playful glint returning to her expression. "Should you have any reasonable questions of me-" She released him from her hold, at least physically, her hands returning to her lap once more, "-then I shall return the favor and be at least somewhat helpful."

He is all too eager to take the opportunity to talk business, to pretend the past few moments didn't happen, that he didn't almost reciprocate to her wiles. "What about the name of your superior?"

She chuckles. "I said I'd be helpful, but not suicidal."

"I'd protect you." He says quickly. Too quickly.

She is already walking towards her chair. "You couldn't protect me at all."

There is a sickly saccharine edge to her words that makes his insides twist with an obscure kind of unease that he can't identify. The only thing he can settle on now is that perhaps now, he knows her a little better, the kind of person she is, the kind of trap she is.

He checks on her later that night when she is asleep, huddled to herself on the chair. She is whimpering in her sleep.

"...don't want...it...don't...want...no..."

And just like that, she's a complete stranger once more.

A/N: I'll admit it, this story is freaking fun to write~. There was a lot going on in this chapter for sure, and it's only going to keep getting more involved! I really hope you enjoyed this chapter. Thank you to everyone still reading thus far, and do leave a review if you feel like it!


	4. Day Four

**Day ?**

She is nine years old again, and terrified.

Her eye-or rather, where it used to be-still hurts. The bandage is always wet, and it's the only thing she's been wearing for the past week.

The room has a smell that scares her.

"Miku."

Rin bows her head. She's learned the hard way to not speak until told to. The untreated welts decorating her body from the past few days remind her.

"That is the name you will call today." She smooths a hand over her cheek in a gesture resembling affection. "Clients like those sort of personal touches. And maybe when you fill out a little more, I'll send you on a few more difficult errands." Miku narrows her eyes at the quivering Rin from behind her glasses. "You still need work though. Lots of it." She takes a seat in the plush, cushioned chair beside the equally lavish bed, crossing her legs and propping her chin on her hand in bemusement. "How about you start off today by...licking my feet."

She does as she is told.

"Now my legs."

She does as she is told.

"Deeper." Miku spreads her legs and smiles.

She hesitates, but does as she is told, trying not to whimper when sharp fingernails dig in to her scalp and draw her suffocatingly close. She is disgusted with herself, who she is now, what she does. Somehow, she has a frayed thread of sanity left by the time Miku finishes.

"Lick your lips."

She does-

"Swallow."

-as she is told.

The hand in her hair yanks until she is to her feet.

"Seduce me."

She doesn't know what to do, what it means, but she's afraid to ask, and even more afraid of not doing anything at all-

"It's your eye that's missing, not your ears, my little Rin."

She hates that Miku knows her name, that she has made her hate it, hate being the person it belongs to.

Miku giggles derisively. "Do you need my help?"

Trembling, she nods. Miku pulls her into her lap and thrusts her tongue into her mouth.

She wants to scream, pull away, purge the rising bile in her throat from her body, but that won't keep her alive, won't give her a chance to find her brother. She misses him so badly.

Miku stands, holding her small body against hers, shoves her deeply into the poison purple and bloody red of the velvet bedding.

"Tell me you want me."

Her mind is screaming for breath and freedom where her lips are unable, and when Miku finally gives her the space, someone who is not Rin answers.

"I w-want you, I want you-" She gasps, forcing herself to continue, to just think of her brother and how badly she misses him and wishes to be held in his arms again, "-please, please-" She returns Miku's kisses and grabs her shoulders, arching to hide her wet eyes as she does things that make her tender, raw body ache and burn beneath her.

As she ruins her.

**Day Four**

She wakes, a bead of sweat dripping off the peak of her nose, and she swallows. Breathing again, she looks up.

Len is standing by her door, and she's fully aware that he was watching everything. She can also tell that he knew better than to wake her and wound her pride more than it already has been.

Begrudgingly, she admits that he has done her yet another service.

Wordlessly he goes to the table and pours out a cup of water, brings it to her, and it is then that she realizes how thirsty she was. She drinks obediently, and the silence is broken only by the sound of her swallowing.

She glances up at him curiously and breaks eye contact again almost as quickly, as unpleasant memories resurface, of her with her hands behind her back, before a man, before many men, not unlike how she is now.

She has been holding her breath, and as she realizes it, she gasps shakily and the cup's contents spill over her, from her collar down the front of her dress.

He asks if she's okay, and before she can lie, she doubles over and retches everything back up.

**A/N: **Wow I can not apologize enough for how long this took, I am so so sorry! I've had a really, really busy year/everything and I feel super awful about making people wait so long, but rest assured I haven't forgotten this fic! I hope you still enjoy it!


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